Thin provisioning (TP), a virtualized storage technology that creates an allocated amount of pooled disk space that apps share, is supported by most array vendors. One of the biggest advantages of TP is that it lets the storage admin increase capacity utilization and not overprovision storage.

But not all TP implementations are alike. Because of intense vendor marketing that emphasizes each company's "superior" implementation, it's difficult for a user to evaluate and compare products. Ashish Nadkarni, principal consultant at Framingham, MA-based GlassHouse Technologies, says users should consider the following when comparing TP offerings:

1. What's the maximum supported pool size? What seems like a fairly straightforward question was answered differently by all of the vendors contacted for this article. "For LUNs, which have been thin provisioned, the maximum LUN size is 1TB; for flexible volumes it is 16TBs," writes Rich Clifton, NetApp's senior VP and general manager, virtualization and grid infrastructures business units, in an email.
   
   Craig Nunes, VP of marketing at 3PAR, writes that InServ Storage Server (600TB in the S800 model), unlike other thin-provisioning implementations, "doesn't require capacity to be pre-dedicated into separate thin-provisioned pools ... as writes occur to any thin-provisioned volume, capacity is sourced and configured on-demand from the entire free space of the array."



2. What's the chunk size? Jerome Wendt, lead analyst and president at Omaha, NE-based DCIG Inc., says the more granular the chunk size, the less overprovisioning will occur. But as small chunks increase, so does the index managing the chunks, which can impact system performance and require more time to manage. Bob Wambach, EMC's senior director, Symmetrix marketing, says "generally, the granularity of chunks allocated from thin pools will not be a significant differentiator in terms of capacity utilization."
   
   Nunes disagrees: "When the unit of capacity consumption is much greater than the size of the write, the efficiencies of thin provisioning are diminished or lost altogether."



3. What's the typical oversubscription ratio? In TP, a large amount of storage is set aside for an app, but storage is only released incrementally as additional capacity is needed; that total ...
   
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   capacity is said to be "oversubscribed." Rob Commins, director of product marketing at Pillar Data Systems, says "aggressive users oversubscribe their system between six and 12 months ahead of their consumption curve."
   
   "As a rule of thumb," adds DCIG's Wendt, "the typical oversubscription ratio is 3:1 on Windows and Linux servers, and 2:1 on DAS."

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